The invention pertains to polyolefin resin compositions that are designed to produce clear films that have satisfactory antiblocking capability and where the antiblocking agent exhibits low abrasivity. These films would be used in a broad range of packaging and covering film applications.
Polyolefin films are used extensively for packaging around the world and, increasingly, are replacing traditional materials such as paper. High clarity polyolefin films allow easy viewing and identification of the package contents. When plastic film is produced, however, there is a tendency for two or more contacting layers of the film to stick together, or "block", making separation of the film, opening the bag, or finding the end of the roll difficult.
Addition of inorganic mineral fillers to polyolefin film to reduce blocking is required commercially. It is well recognized that films produced from resin containing antiblock fillers have a rougher surface, which reduces the intimate contact between layers of film and reduces blocking, hence, the term "antiblocking agent" is applied to such fillers.
Not all inorganic fillers are effective antiblocks and some effective antiblocks have other problems (such as high cost, high abrasivity, adverse affect on opticals, health hazard) which limit their commercial utility. The objective is to add as little antiblock as possible to reduce blocking force to the required level, while minimizing the adverse effects on optical properties of the film and other concerns such as wear on processing equipment.
Diatomaceous earth has been widely used as a moderately effective antiblocking agent but has the following adverse attributes: fair film haze, poor film clarity, very high abrasiveness, and is moderately expensive. Talc is also widely used in certain polyolefin formulations as a moderately effective antiblocking agent. Its advantages over diatomaceous earth are lower cost, excellent film clarity, and very low abrasiveness. However, its film haze is usually only fair and would not be suitable for high clarity packaging applications. While nepheline syenite or feldspar have been considered as antiblocking agents for high clarity film applications (because their optical index of refraction is closer to that of polyethylene), they are relatively ineffective in reducing blocking forces, and have very high abrasiveness.
Abrasivity of inorganic antiblocks is of concern for several reasons. Highly abrasive antiblocks will contribute to rapid equipment wear in compounding and processing equipment. When wear reaches the point where it changes the equipment dimensions in critical areas, both dispersion of additives in the resin and output rates can be adversely affected. In such cases, product quality may suffer and production costs may increase, particularly if the equipment must be taken out of service and new parts purchased to replace worn pieces of equipment. In addition, abrasion of equipment will introduce metal contamination into the plastic product, which may have a detrimental effect on product stability or color or both. Antiblocks with low abrasiveness are preferred for these reasons.
There have been many attempts by others to solve the problem of balancing polyolefin film antiblocking and haze properties but none have addressed the additional concerns of film clarity and the antiblock abrasivity and cost (which are all necessary considerations for a viable commercial product). The existing situation is that a cost effective antiblocking formulation for high clarity polyolefin films with low abrasivity of the antiblock has not yet been found.